Turbulent Mixing and the Formation of an Intermediate Nepheloid Layer Above the Siberian Continental Shelf Break

Abstract

Intermediate nepheloid layers (INLs) form important pathways for the cross‐slope transport and vertical export of particulate matter, including carbon. While intermediate maxima in particle settling fluxes have been reported in the Eurasian Basin of the Arctic Ocean, direct observations of turbid INLs above the continental slope are still lacking. In this study, we provide the first direct evidence of an INL, coinciding with enhanced mid‐water turbulent dissipation rates, over the Laptev Sea continental slope in summer 2018. Current velocity data show a period of enhanced downslope flow with depressed isopcynals, suggesting that the enhanced turbulent dissipation is probably the consequence of the presence of an unsteady lee wave. Similar events occur mostly during ice free periods, suggesting an increasing frequency of episodic cross‐slope particle transport in the future. The discovery of the INL and the episodic generation mechanism provide new insights into particle transport dynamics in this rapidly changing environment.

Increasing Nutrient Fluxes and Mixing Regime Changes in the Eastern Arctic Ocean

“…Furthermore, in the near-bottom layer of the easternmost shelf stations (indicated with a green circle and green dots in Figures 1a and 1d) nitrate concentrations were high (up to 10 mmol m −3 ) in 2018. An unusual vertical nitrate distribution was found at two stations above the continental slope (transect V, 2018, green arrow in Figures 1a and 1b), a consequence of previously identified strong local vertical mixing at this location (Schulz, Büttner, et al, 2021, discussed in detail in Section 5).…”

“…Their contribution to vertical nitrate supply may be approximately equal to the steady, but weak turbulent fluxes over the entire Siberian Sea area. Similar events have probably taken place in the past, but their occurrence has been linked to ice‐free conditions in the Laptev Sea, and their frequency has likely increased with progressing sea ice reduction (Schulz, Büttner, et al., 2021). Assuming that this trend continues, we may expect a further increase in the relative importance of episodic boundary mixing events to the total Pan‐Arctic vertical nutrient supply.…”

“…Mid‐water turbulent dissipation rates at this station are up to two orders of magnitude larger than the typical values of 10 −9 W kg −1 found in this region (Schulz, Janout, et al., 2021). This intense mixing episode has been attributed to the dissipation of an unsteady lee wave associated with a passing continental shelf wave linked to a farfield storm (Schulz, Büttner, et al., 2021). The strong mixing episode redistributed nitrate, dissolved oxygen, fluorescent material, temperature and salinity (Figures 3c–3g, only the relevant upper 200 m are shown), leading to homogeneous concentrations between 30 and 300 m water depth.…”

“…While enhanced mid‐water dissipation attributed to tidally‐generated unsteady lee waves has been observed over Arctic continental slopes (Fer et al., 2015, 2020; Padman et al., 1992; Rippeth et al., 2017), the cross‐slope tidal flow in the Siberian Seas is weak, with low levels of tidal conversion and mixing (Fer et al., 2020; Rippeth et al., 2015). Energy conversion mechanisms to generate enhanced mixing from larger scale flow anomalies have just recently been described (Schulz, Büttner, et al., 2021). In addition, over the past decade and a half, warming (Barton et al., 2018) and shoaling of the AW (Polyakov et al., 2017, 2020) together with a weakening of the overlying halocline have led to significantly increased ventilation of the AW (Polyakov et al., 2020; Schulz, Janout, et al., 2021).…”

Changes in Arctic Halocline Waters Along the East Siberian Slope and in the Makarov Basin From 2007 to 2020

“…The transition zone between a cold and warm LH is believed to be located near the slope of the East Siberian shelf, although still little documented (Jung et al., 2021; Wang et al., 2021). Warmer LH water can result from diapycnal mixing between upwelled AW and bottom shelf waters, as well as enhanced vertical mixing of the LH with AW over sloped topography (Bauch et al., 2016; Fer et al., 2020; Schulz et al., 2021a; Wang et al., 2021; Woodgate et al., 2005). The upper halocline (UH; green layer in Figure 1d) receives relatively fresh and cold shelf waters—comprising river runoff and Pacific‐origin waters—and exhibits different hydrographic/biogeochemical characteristics in the Eurasian, Makarov, or Canada Basins (e.g., Rudels et al., 2015; Swift et al., 1997).…”